The present invention relates to a ceramic substrate, particularly to an ceramic circuit substrate for use in electronic circuits, etc., and a method for forming the same.
With recent higher integration density and higher speed of LSI chips, etc., the LSI chips, etc. give forth or discharge more heat, and it is increasingly required to positively cool the electronic parts. In the conventional LSI chips or packages, etc., heat radiation fins are provided in the devices and their substrates for cooling, the substrates themselves are formed of highly heat conducting materials, and channels for coolants are provided in the circuit substrates, whereby their substrate temperatures are lowered as much as possible.
For example, Japanese Patent Laid-Open Publication No. 52-12524/1977 discloses a liquid cooling electronic device in which a region on a circuit substrate for a device is defined by a spacer, and the region defined by the spacer is filled with a coolant, whereby the device is cooled.
Japanese Patent Laid-Open Publication No. 62-252198/1988 and Japanese Patent Laid-Open Publication No. 60-126854/1985 disclose cooling apparatus in which cooling fluid is injected from nozzles to electronic parts on circuit substrates to supply the cooling fluid directly to the individual electronic parts, whereby the electronic parts are cooled.
Japanese Patent Laid-Open Publication No. 61-92721/1986 discloses a method for making a large cooling panel by burying a core in a groove formed in a substrate, welding a top plate, and removing the core.
Japanese Patent Laid-Open Publication No. 61-154098/1986, Japanese Patent Laid-Open Publication No. 03-225889/1991 and Japanese Patent Laid-Open Publication No. 03-242996/1991 disclose circuit substrates in which coolant channels are provided in the circuit substrates to prevent temperature rises in electronic circuits.
Japanese Patent Laid-Open Publication No. 03-263398/1991 discloses a cooling structure for cooling a circuit substrate, which comprises a cooling plate provided in a heating module opposed to the circuit substrate.
In connection with forming the channel inside a circuit substrate the following art are proposed. That is, in preparing the substrate by green sheet lamination, a pattern of an organic substance is formed on the surfaces of green sheets by screen printing, and a number of the green sheets are laminated, and then sintered to decompose and scatter or liberate the organic substance to form the channel, or grooves are formed in green sheets in the middle parts, and the green sheets are laminated and sintered to form a channel.
But the above-described conventional art need large-sized cooling means on the circuit substrates, cooling pipes for coolants to flow, and cooling plates, which make the circuit substrate large, and make the structures complicated.
The proposed green sheet lamination method for forming the channel in a circuit substrate has problems that time is taken to decompose and scatter the organic substance to form the channel, and the resultant channel has a complicated configuration which results in high flow resistance. The formation of the channel by laminating grooved green sheets has a problem that the grooves might be broken under pressure when the green sheets are laminated, with the result of higher flow resistance. Higher flow resistance makes flow of the coolant difficult with the result of lower cooling effect.